This invention relates to a brush assembly for use with a commutator of a dynamoelectric machine.
FIGS. 1 and 2 illustrate one example of a conventional brush assembly for a dynamoelectric machine in which an annular base plate 4 made of an electrically conductive plate is provided around a commutator 2 of a dynamoelectric machine. The base plate 4 can be directly mounted to the frame (not shown) of the dynamoelectric machine by a suitable fastening means such as screws (not shown) extending through holes 6 formed in the base plate 4. Thus, a direct electrical connection is established between the machine frame and the base plate 4. The base plate 4 has secured thereon a pair of brush holders 8a and 8b for slidably holding therein brush elements 10a and 10b, respectively. The brush elements 10a and 10b are in electrical contact with the brush holders 8a and 8b, and the holders 8a and 8b are electrically connected to the base plate 4 by means of lead conductors 12a and 12b. Thus, the brush elements 10a and 10b are used as brushes for the minus side. In order to establish a good electrical contact between the brush elements 10a and 10b and the communtator 2, brush springs 14 are disposed between the base plate 4 and the radially outer ends of the brush elements 10a and 10b.
The base plate 4 is also provided thereon with a pair of plus side brush elements 16a and 16b which are radially slidably supported by the brush holders 18a and 18b and biased toward the commutator 2 of the dynamoelectric machine by means of springs 20. In order to electrically insulate the brush holders 18a and 18b from the base plate 4, the brush holders 18a and 18b are mounted on the base plate 4 with electrically insulating plastic sheets 22a and 22b interposed therebetween as best seen in FIG. 2. It can be seen from FIG. 2 that the brush holder 18a is secured to the insulating sheet 22a by grommets 24, and the insulating sheet 22a is secured to the base plate 4 by means of the screws (not shown) for mounting the base plate to the machine frame. The brush holders 18a and 18b are electrically connected through lead conductors 26a and 26b to one end of the lead 28 the other end of which is connected to the terminal plate 30.
With the above structure, two electrically insulating plastic sheets 22a and 22b are necessary in order to electrically insulate the plus side brush elements 16a and 16b as well as the brush holders 18a and 18b from the base plate 4. Also, since the grommets 24 are used, time-consuming operations for forming the grommets 24 must be carried out, and there is the risk of damaging the insulating plastic sheets 22a and 22b during the grommet forming operation.